Most of those things you mention are in our package from the trust. You will not get any of the services for nothing!!! It all costs. We have insurance for staff absence. Buildings are insured centrally.
Services have not disappeared by 80%. Schools have been given the funds via the formula to buy the services. We buy them from our LA Trust. Schools can buy from other providers if they wish. There are other providers in the market place.
The LA formed the Trust to provide services to schools. It covers finance, HR, payroll, school improvement and advice, buildings, Sen advice, CPD, Governor services, bursar support, it support, sports advice, etc etc.
Our DH does all our supply. If a mat has floating staff, the schools are paying for it. No service is ever free but our trust works like a mat but it’s the LA that formed it to provide continuity. That’s why we have few primary academies. Little point.
A mat will withhold money from the schools to pay for central services or they will devolve finance to the schools and you buy in services from them. There is no magic model that gives you more money and free services!
I assume that not all LAs have not bothered to set up a Trust so you don’t get the services from them but you have the devolved money. It’s a big mistake for LAs not to have done this for all sorts of reasons.